1. Field of the Invention
The present invention is directed to a circularly polarized antenna arrangement for use as a local antenna arrangement in a nuclear magnetic resonance tomography apparatus, and to a method for operating such a local coil arrangement.
2. Description of the Prior Art
The signal-to-noise ratio in MR imaging can be noticeably improved with circularly polarized antennas compared to linearly polarized antennas, given the same exposure time. Circularly polarized whole-body antennas can be realized in a simple way by arranging two sub-antennas offset by 90.degree. at the circumference of a cylindrical examination space.
U.S. Pat. No. 4,816,765 shows a local coil having a flat structure that likewise generates a circularly polarized field. A so-called butterfly coil arrangement is used to generate one field component lying parallel to the local coil in the examination region. A field component is also generated perpendicular to the surface of the surface coil by a further coil lying in the middle of the butterfly arrangement.
Antenna arrays, i.e. a chain of a plurality of sub-antennas, are often employed, for example for examining the spinal column or for the examination of transversal body layers. Such an array is disclosed for example, in PCT application WO 89/05115. This array, however, is linearly polarized and thus has a correspondingly poorer signal-to-noise ratio.
Due to the coupling between the individual coils that arises, the circularly polarized surface coil disclosed by the aforementioned U.S. Pat. No. 4,816,765 is not suitable for arrays that extend transversely relative to the basic magnetic field.